The present invention relates in general to optical communication systems and components therefor, and is particularly directed to a new and improved, tunable micro-optic architecture for combining non-collimated, orthogonally polarized light beams transported over polarization maintaining (PM) optical fibers, whose mutual spatial separation may vary, as in the case of fibers contained in a dual capillary structure, so as to produce a single composite output beam that may be readily coupled to a downstream single beam processing device, such as a Raman amplifier.
A variety of optical signal processing applications require amplification of one or more optical information beams, such as a pair of laser beams transported over a dual fiber supporting capillary. Non-limiting examples of a dual optical fiber capillary structure are shown in the diagrammatic cross sectional views of FIGS. 1, 2 and 3, which depict respective rectangular, dual circular and xe2x80x98FIG. 8xe2x80x99 configurations. As shown therein, each dual fiber capillary has a pair of optical fiber-supporting bores 11 and 12, in which associated optical fibers are fixedly retained by a suitable adhesive (e.g., epoxy) 13 within a shaped bore 14 of a surrounding glass capillary 15. In this type of structure, it is common practice to transport beams of different polarizations over the respective fibers installed within the bores 11 and 12.
For this purpose, each fiber may comprise a polarization-maintaining (PM) fiber structure, shown in cross-section in FIG. 4 as having a signal-transporting glass core 41, embedded at a central axis 42 within a surrounding cladding 43. As a non-limiting example, the fiber""s central core 41 may have a diameter on the order of 10 microns (with an on-central axis tolerance on the one micron), while the surrounding cladding 43 may have a diameter on the order of 125 microns (+/xe2x88x92three microns). Embedded within the cladding 43 and equidistantly spaced apart along the central glass core""s xe2x80x98slowxe2x80x99 axis 44 (which is orthogonal to its fast axis 45) are a pair of stress rods 46 and 47 (typically referred to xe2x80x98Panda eyes). These stress rods are used to introduce birefringence into the core area so as to maintain the polarization of the light beam being transported along the central core 41.
In an effort to maximize processing or interfacing flexibility for the two beams being transported by a dual fiber capillary structure with a downstream single beam-based device, such as but not limited to a beam combiner for a Raman amplifier, a pair of PM fibers are typically installed in the two fiber-supporting bores of a dual capillary structure, such that their Panda-eyes and therefore the associated polarization directions of the (laser) beams in the cores are mutually orthogonal to one another, as diagrammatically illustrated at 48 and 49 in FIG. 5.
Now although such a mutually orthogonal, intra-capillary fiber orientation provides the desired difference in polarization in the respective fiber cores, efficiently combining the beams carried by the cores depends on how well the parameters of a combining device coupled thereto is able to accommodate fiber-to-fiber separation between the cores, which varies not only with capillary separation, but also with differences in the parameters of the Panda-eyed fiber structures epoxied within the two bores. Namely, if the beam combiner is designed for a fiber separation that is different from that of the dual capillary structure with which the combiner is actually used, the degree of spatial overlap of the two beams along a combining axis will be degraded, which can result in a substantial loss in one or both polarization components of the composite output beam.
Pursuant to the invention, this potential misalignment problem is successfully addressed by a new and improved, tunable PM fiber combiner, which is configured to be accurately alignable with and is operative to combine into a single composite beam a pair of non-collimated, orthogonally polarized light beams transported over polarization maintaining (PM) optical fibers, whose mutual spatial separation may vary. The resulting composite light beam may then be readily coupled (e.g. via a single mode fiber) to a downstream unitary beam processing device, such as a Raman amplifier.
In a first embodiment, a pair of birefringent walk-off crystal wedges that form a generally xe2x80x98rectangularxe2x80x99 combiner are cascaded along the beam travel directions of mutually polarized light beams output from respective fibers of a dual fiber capillary, such as a dual xe2x80x98Panda-eyedxe2x80x99 capillary structure. The two wedges are displaceable relative to one another in directions either generally transverse or parallel to the optical beam path, so that the combined walk-off distance of the crystal wedges is adjustable, in order to xe2x80x98tunexe2x80x99 the effective optical path length of the wedges, and accommodate variations in fiber core-to-fiber core separation within a dual xe2x80x98Panda-eyedxe2x80x99 capillary structure. In a second embodiment, a pair of birefringent walk-off crystal wedges are cascaded into a generally xe2x80x98non-rectangularxe2x80x99 tunable combiner.
A third embodiment employs a pair of fixed length, angularly adjustable, walk-off 45xc2x0 crystals, that are arranged to intercept a pair of light beams from a dual capillary optical fiber structure having mutually orthogonal Panda-eyes spatially oriented at +45xc2x0 and xe2x88x9245xc2x0 relative to a line passing through their respective fiber cores. The effect of this mutual +/xe2x88x9245xc2x0 angular spatial orientation of the two fibers is to have their slow axes intersect at 90xc2x0 at a location within the cladding of the dual fiber capillary proximate to but slightly spaced apart from the two fibers. For a nominal separation Do between the fiber cores, the walk-off distance required to achieve aligned overlap of the mutually orthogonal beams within the cores is on the order of 1.414*Do. Each walk-off crystal has a thickness that provides a nominal walk-off of 1.414*Do for a prescribed beam polarization orientation relative to respective walk-off axes between beam input and exit output faces.
The adjustability of the angular orientations of the crystals about the optical axes of the two beams incident on the crystals compensates for a departure in actual fiber-to-fiber separation Do from the nominal value of Do for different dual capillary structures. For a larger than nominal separation (D greater than Do), the crystals may be rotated to increase the angle between their walk-off axes and a line through their cores to values larger than +/xe2x88x9245xc2x0, and thereby realize exact coincident overlap of the two mutually orthogonally polarized light beams emanating from the fiber cores at an increased distance walk-off axis intersection location. For a smaller than nominal separation (D less than Do), the crystals may be rotated to decrease the angle between their walk-off axes and a line through their cores to values smaller than +/xe2x88x9245xc2x0, so as to cause exact coincident overlap of the two mutually orthogonally polarized light beams emanating from the fiber cores at a relatively closer than nominal walk-off axis intersection location.
A fourth embodiment of the invention employs a Wollaston prism, which is coupled to the dual capillary PM fiber by way of a lens, such as a gradient refractive index (GRIN) lens and a glass spacer. The glass spacer may be bonded to either the terminating face of the PM capillary or the GRIN lens. The two beams from the dual fiber capillary are directed by the GRIN lens at respective complementary angles of divergence into the Wollaston prism, depending upon the pitch of the GRIN lens. For a given core-to-core spacing for mutually orthogonally oriented fibers, and a GRIN lens having a prescribed pitch, the divergence angle of each beam from the GRIN lens may be readily calculated and converted by the Wollaston prism into a composite output beam containing both polarizations. Optically downstream of the Wollaston prism is a further GRIN lens and associated glass spacer that couples the output of the GRIN lens into an output fiber coupler terminating an individual PM or single mode fiber.